The present invention relates to a system for insulating and finishing the exterior of a building.
Rain penetration is one of the oldest problems building owners have had to deal with yet it still occurs all too frequently. The penetration of rain not only can damage interior finishes and materials but it can also damage the structure of the walls themselves.
Rain penetration results when a combination exists of water at the surface of the wall, openings through which it can pass, and a force to move the water through these openings. The elimination of any one of these three conditions could prevent the occurrence of rain penetration. While wide roof overhangs may help to shelter the walls of a low-rise building, similar protection is not available to higher buildings. Therefore, one of the remaining two conditions must be eliminated to prevent rain penetration.
The face seal approach attempts to eliminate all the openings in the wall through which water can pass. However, the materials used to seal all these openings are exposed to extremes of weather and to movements of the building. Even if the problems of job site inaccuracies and poor workmanship can be overcome and a perfect seal can be achieved, the in-service weather conditions may eventually cause the deterioration and failure of these seals, creating openings in the wall through which water can pass. Unfortunately, these openings can be extremely tiny and difficult to identify, so that even an extensive maintenance program may not keep the building free of openings.
The alternate approach to controlling rain penetration is to eliminate the forces which drive or draw water into the wall. There are typically considered to be four such forces: kinetic energy, capillarity, gravity and wind pressure differences.
For a wind-driven rain storm, rain droplets can be blown directly into large openings in the wall. However, if there is no direct path to the interior, the rain droplets will not pass deeply into the wall. Where large openings, such as joints, are unavoidable, the use of battens, splines, baffles or overlaps has been successful in minimizing rain penetration caused by the kinetic energy of the rain drops.
Due to the surface tension of water, voids in a material will tend to draw in a certain amount of moisture until the material approaches saturation. If capillaries pass from the exterior to the interior, water can move through the wall due to the action of capillary suction. While partial water penetration of a wall by capillarity is characteristic of porous cladding material, the introduction of a discontinuity or air gap can prevent through-wall movement of water.
The force of gravity will cause water to move down the face of the wall and into any downward sloped passages into the wall. To prevent gravity induced movement through joints, they are typically designed to slope upwards from the exterior. Unintentional cracks or openings are more difficult to control. If there is a cavity directly behind the exterior face of the wall, any water that does flow through the wall will then be directed downward, by gravity, on the inboard face of the exterior wall. At the bottom of the cavity, the water can then be drained back to the outside through the use of sloped flashings.
An air pressure difference across the wall of a building is created by stack effect, wind and/or mechanical ventilation. If the pressure on the exterior face of the wall is higher than on the interior of the wall, water can be forced through tiny openings in the wall. Research has shown that the amount of rain moved through the cladding by this mechanism is the most significant. It has previously been recognized that this force can be eliminated or reduced by the use of the pressure-equalized cavity.
The theory of the pressure equalized cladding is that it neutralizes the air pressure difference across the cladding (caused by wind) which causes water penetration. It is impossible to prevent wind from blowing on a building but it is possible to counteract the pressure of the wind so that the pressure difference across the exterior cladding of the wall is close to zero. If the pressure difference across the cladding is zero, one of the main forces of rain penetration is eliminated.
In previous proposals, a rainscreen wall incorporates two layers or wythes separated by an air space or cavity. The outer layer or cladding is vented to the outside. When wind blows on the building facade, a pressure difference is created across the cladding. However, if the cavity behind the cladding is vented to the outside, some of the wind blowing on the wall enters the cavity, causing the pressure in the cavity to increase until it equals the exterior pressure. This concept of pressure equalization presupposes that the inner wythe of the wall is airtight. This inner wythe, which includes an air barrier, must be capable of sustaining the wind loads in order for pressure equalization to occur. If there are significant openings in the air barrier, the pressure in the cavity will not equalize and rain penetration may occur.
More recently, it has been recognized that optimum insulation of a building is obtained if the insulating material is applied to the exterior of the building. With the insulation on the outside of the building, thermal bridges due to structural components of the building are eliminated and a consistently high R value is provided.
The application of external insulation to a rainscreen wall has, however, led to practical difficulties due to the need to provide for the equalization of pressure within the cavity defined by the insulation and still comply with model building codes. The spacing of the insulation from either the load bearing structure or the cladding to define the cavity leaves one face of the insulation exposed. This is contrary to model building codes, such as, for example, the National Building Code of Canada (NBCC) which requires that combustible insulation must have all faces sealed. Therefore, this type of construction can only be used in applications that permit combustible construction, typically building under three stories high. As a result, external insulation has been used with face seal systems and rainscreen walls have been used with internal insulation.